Batteries used in electronic devices are often made with multiple cells, each having a lower voltage than the entire battery, which are connected in series to provide the desired voltage for the battery. For example, a 12V battery may include ten 1.2V cells stacked in series to add up to the desired output voltage. Because modern batteries have a very high energy density, they present a danger of overheating and explosion if defective or if not used correctly. Devices powered by batteries often include monitoring circuits to protect the device and/or the battery. Various monitoring circuits may detect conditions such as defective batteries generating too much or too little current or voltage, defective electrical connections between a battery and a device, the use of improper battery chargers supplying too much current, etc. Monitoring circuits may be used to warn a user or to turn off the device.
Power supply monitoring circuits designed for a single voltage input or single cell battery are not as effective at protecting devices and batteries when the battery includes multiple cells. For example, two adjacent cells in the battery may short together, creating a potentially hazardous condition, while the overall battery voltage remains substantially unchanged. In this case, a monitoring circuit designed for a single cell battery would monitor only the overall battery voltage and would not immediately detect the hazardous internal short circuit.
The design of monitoring circuits for multi-cell batteries is complicated by the fact that each cell in a stack operates at a different voltage level. For example, in the ten cell 12V battery above, the output of the bottom cell when fully charged is 1.2V, and the output of the ninth cell up the stack is 10.8V. Thus, monitoring circuits must typically be customized for the various voltage levels in a stack of cells in a multi-cell battery.